System and method for multiplexing gear engagement control and providing fault protection in a toroidal traction drive automatic transmission

ABSTRACT

An apparatus for multiplexing gear engagement control in an automatic transmission is provided. At least two friction engagement devices are configured to selectively engage and disengage a different gear ratio of the transmission. A trim system is configured to selectively supply engagement and disengagement pressures to at least one fluid passageway. A first control valve is fluidly coupled directly to the at least one fluid passageway and directly to each of the at least two friction engagement devices. The first control valve is configured to selectively route the engagement and disengagement pressures through the first control valve directly to the at least two friction devices.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/957,877, entitled “SYSTEM AND METHOD FOR MULTIPLEXING GEARENGAGEMENT CONTROL AND PROVIDING FAULT PROTECTION IN A TORODIAL TRACTIONDRIVE AUTOMATIC TRANSMISSION,” which was filed on Dec. 1, 2010, andwhich claims priority to U.S. Provisional Patent Application No.61/287,038, filed Dec. 16, 2009, the entirety of both of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to continuously variabletransmissions including a variator, and more specifically to systems andmethods for multiplexing gear engagement control and providing faultprotection in such transmissions.

BACKGROUND

Toroidal traction drive automatic transmissions may include a variator,one or more gear sets and a number of selectively engageable frictiondevices that cooperate together to transfer drive torque from a powerplant to one or more loads. It is desirable to multiplex gear engagementcontrol in such transmissions, and to provide fault protection for oneor more faults or failure conditions.

SUMMARY

The present invention may comprise one or more of the features recitedin the attached claims, and/or one or more of the following features andcombinations thereof. An apparatus for multiplexing gear engagementcontrol in an automatic transmission may comprise at least two frictionengagement devices each configured to selectively engage and disengage adifferent gear ratio of the transmission, a trim system configured toselectively supply engagement and disengagement pressures to at leastone fluid passageway, and a first control valve fluidly coupled directlyto the at least one fluid passageway and directly to each of the atleast two friction engagement devices. The first control valve may beconfigured to selectively route the engagement and disengagementpressures through the first control valve directly to the at least twofriction devices.

The at least one fluid passageway may comprise a first fluid passagewayand a second fluid passageway separate from the first fluid passageway.The trim system may be configured to selectively supply the engagementand disengagement pressures to the first fluid passageway and toselectively supply the engagement and disengagement pressures to thesecond fluid passageway independently from the first fluid passageway.The first control valve may be fluidly coupled directly to the firstfluid passageway.

The apparatus may further comprise a second control valve fluidlycoupled directly to each of the first and second fluid passageways. Thesecond control valve may be configured to selectively route theengagement and disengagement pressures in the first and second fluidpassageways through the second control valve to the first control valvefor further selective routing by the first control valve to each of theat least two friction devices.

The apparatus may further comprise a third friction device configured toselectively engage and disengage another different gear ratio of thetransmission. The second control valve may be fluidly coupled directlyto the third friction device. The second control valve may be configuredto selectively route the engagement and disengagement pressures in thesecond fluid passageway through the second control valve directly to thethird friction device.

The first control valve is configured to selectively route theengagement and disengagement pressures in the first fluid passagewaythrough the first control valve to the second control valve for furtherselective routing by the second control valve to the third frictiondevice.

The first control valve may comprise a first spool having a strokedposition and a de-stroked position and the second control valve maycomprise a second spool having a stroked position and a de-strokedposition. The first and second valves may together be configured tosupply the engagement pressure in at least one of the first and secondfluid passageways to at least one of the three friction engagementdevices to thereby engage the at least one of the three frictionengagement devices in all possible position combinations of the firstand second spools.

The second control valve may be fluidly coupled directly to a first mainpressure fluid passageway and also directly to a third fluid passageway.The second control valve may be configured to selectively route pressurein the first main pressure fluid passageway to the third fluidpassageway. The second control valve may be fluidly coupled directly toa second main pressure fluid passageway. The second control valve may beconfigured to selectively route pressure in the second main pressurefluid passageway to the third fluid passageway.

The automatic transmission may be a toroidal traction drivetransmission. The toroidal traction drive transmission may comprise avariator and a variator control system for controlling operation of thevariator. The second control valve may be configured to selectivelyroute pressure in the first and second main pressure fluid passagewaysto a component of the variator control system via the third fluidpassageway.

An apparatus for multiplexing gear engagement control in an automatictransmission may comprise three friction engagement devices eachconfigured to selectively engage and disengage a different gear ratio ofthe transmission, a trim system configured to selectively supplyengagement and disengagement pressures to at least one fluid passageway,a first control valve fluidly coupled directly to the at least one fluidpassageway and directly to each of two of the three friction engagementdevices, and a second control valve fluidly coupled directly to the atleast one fluid passageway and directly coupled to the third frictiondevice. The first control valve may be configured to selectively routethe engagement and disengagement pressures through the first controlvalve directly to the at least two friction devices. The second controlvalve may be configured to selectively route the engagement anddisengagement pressures through the second control valve directly to thethird friction device.

The first and second control valves may each include an actuatorresponsive to a separate control signal to independently control thefirst and second valves between stroked and de-stroked states to therebydefine four separate combinations of operating states of the first andsecond valves.

The transmission may define three different operating modes with adifferent one of the three friction devices engaged during each of thethree different operating modes.

The at least one fluid passageway may comprise a first fluid passagewayand a second fluid passageway separate from the first fluid passageway.The trim system may be configured to supply the engagement pressures toeach of the first and second fluid passageways during transitionsbetween the three different operating modes of the transmission tothereby engage two of the three friction engagement devices during thetransitions.

Two of the four separate combinations of operating states of the firstand second control valves may be possible during normal transitionsbetween the three different operating modes of the transmission. Theremaining two of the four separate combinations of operating states ofthe first and second control valves may represent fault conditions.

The first and second control valves may be configured to route theengagement pressure to at least one of the three friction engagementdevices during the fault conditions to thereby selectively engage atleast one of the different gear ratios of the transmission during thefault conditions.

The first and second control valves may be configured to route theengagement pressure to two of the three friction engagement devicesduring the fault conditions.

An apparatus for multiplexing gear engagement control in a toroidaltraction drive automatic transmission may comprise at least two frictionengagement devices each configured to selectively engage and disengage adifferent gear ratio of the transmission, a trim system configured toselectively supply engagement and disengagement pressures to at leastone fluid passageway, and a first control valve fluidly coupled directlyto the at least one fluid passageway and directly to each of the atleast two friction engagement devices. The first control valve may beconfigured to selectively route the engagement and disengagementpressures through the first control valve directly to the at least twofriction devices. The toroidal traction drive transmission may furthercomprise a variator and a variator control system for controllingoperation of the variator.

The apparatus may further comprise a third friction device configured toselectively engage and disengage another different gear ratio of thetransmission, and a second control valve fluidly coupled directly to theat least one fluid passageway, directly to the third friction device,and directly to a first main pressure fluid passageway. The secondcontrol valve may be configured to selectively route the engagement anddisengagement pressures through the second control valve directly to thethird friction device. The second control valve may further beconfigured to selectively route pressure in the first main pressurefluid passageway to a component of the variator control system.

The second control valve may further be configured to selectively routepressure in a second main pressure fluid passageway to the component ofthe variator control system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one illustrative embodiment of a system forcontrolling operation of a toroidal traction drive automatictransmission.

FIG. 2A is a diagram illustrating operation of one illustrativeembodiment of a variator that forms part of the toroidal traction driveautomatic transmission illustrated in FIG. 1.

FIG. 2B is a diagram further illustrating operation of the variator ofFIG. 2A.

FIG. 3 is a schematic diagram of one illustrative embodiment of theelectro-hydraulic control system that forms part of the toroidaltraction drive automatic transmission illustrated in FIG. 1.

FIG. 4 is a magnified view of the clutch control valves illustrated inFIG. 3 showing one operating state thereof.

FIG. 5 is a magnified view of the clutch control valves illustrated inFIG. 3 showing another operating state thereof.

FIG. 6 is a magnified view of the clutch control valves illustrated inFIG. 3 showing yet another operating state thereof.

FIG. 7 is a magnified view of the clutch control valves illustrated inFIG. 3 showing still another operating state thereof.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to a number of illustrativeembodiments shown in the attached drawings and specific language will beused to describe the same.

Referring now to FIG. 1, a block diagram is shown of one illustrativeembodiment of a system 10 for controlling operation of a toroidaltraction drive automatic transmission 14. In the illustrated embodiment,a power plant or energy center 12 is coupled to an automatictransmission 14 such that a rotatable output shaft 16 of the power plant12 is coupled to a rotatable input shaft 18 of the transmission 14 in aconventional manner. The input shaft 18 is coupled, in the illustratedembodiment, to a combination variator and gear set 20 that furtherincludes a plurality of selectively engageable friction devices, e.g.,one or more conventional, selectively engageable clutches or the like,and an output of the combination variator and gear set 20 is coupled toa rotatable output shaft 22. The combination variator and gear set 20 isillustratively controlled by an electro-hydraulic control system 24,some of the details of which will be described in greater detailhereinafter.

The power plant 12 is generally an apparatus that produces rotationaldrive power at the output shaft 16. Examples of the power plant 12include, but should not be limited to, one or any combination of a oneor more engines, such as an internal combustion engine of the sparkignited, compression ignition or other variety, a steam engine, or typeof engine that produces mechanical energy from one or more other fuelsources, one or more electrical generators, and the like.

The combination variator and gear set 20 illustratively includes aconventional full-toroidal, traction-drive variator that is coupled to aconventional gear set. Referring to FIGS. 2A and 2B, one illustrativeembodiment of some of the structural features of such a full-toroidal,traction-drive variator 40 is shown. In the illustrated embodiment, thevariator 40 includes a pair of opposing, toroidal-shaped disks 42 and 44that rotate independently of each other. For example, the disk 42 isrigidly coupled to the input shaft 18 of the transmission 14 such thatthe disk 42 is rotatably driven by the power plant 12. The disk 44 isrigidly coupled to an output shaft 46 of the variator 40, and isrotatably coupled to the shaft 18 such that the disk 44 rotates freelyabout the shaft 18. The output shaft 46 of the variator 40 is coupleddirectly, or indirectly through one or more transmission gears, to theoutput shaft 22 of the transmission 14 such that output shaft 46 of thevariator 40 drives one or more wheels of a vehicle (not shown) carryingthe power plant 12 and transmission 14.

A number of rollers 48 are illustratively positioned between opposinginner, arcuate-shaped surfaces of the disks 42 and 44, and a tractionfluid (not shown) is disposed between the rolling surface of each suchroller 48 and the inner surfaces of the disks 42 and 44. In theillustrated embodiment, the rolling surfaces of the various rollers 48therefore do not contact, in a structural sense, the inner surface ofeither disk 42, 44; rather torque is transmitted by the various rollers48 between the two disks 42, 44 via the traction fluid. It is becausetorque is transferred between the two disks 42, 44 via the tractionfluid and not via structural contact between the rolling surfaces of therollers 48 and the arcuate inner surfaces of the disks 42, 44 that thevariator is referred to as a traction-drive apparatus.

In the embodiment illustrated in FIGS. 2A and 2B, two such rollers 48 ₁and 48 ₂ are shown operatively positioned between the opposing innersurfaces of the two disks 42, 44. A roller actuator 50 ₁, e.g., in theform of a conventional hydraulically actuated piston, is coupled to theroller 48 ₁ via a bracket 52 ₁, and another roller actuator 50 ₂, e.g.,in the form of another conventional hydraulically actuated piston, iscoupled to the roller 48 ₂ via a bracket 52 ₂. It will be understoodthat the brackets 52 ₁ and 52 ₂ do not represent rotatable shafts aboutwhich the rollers 48 ₁ and 48 ₂ may be rotatably driven. Rather, thebrackets 52 ₁ and 52 ₂ represent structures about which the rollers 48 ₁and 48 ₂ rotate. In one actual implementation, for example, the brackets52 ₁ and 52 ₂ are configured to attach to the central hub of the rollers48 ₁ and 48 ₂ on either side thereof such that the brackets 52 ₁ and 52₂ and actuators 50 ₁ and 50 ₂ would extend generally perpendicular tothe page illustrating FIGS. 2A and 2B.

The hydraulically controlled actuators 50 ₁ and 50 ₂ are eachillustratively controllable, by selectively controlling a high-sidehydraulic pressure applied to one side of the actuator and a low-sidehydraulic pressure applied to the opposite side of the actuator, tothereby control torque transferred from a corresponding roller 48 ₁, 48₂ relative to the inner, annular surfaces of the two disks 42, 44. Theactuators 50 ₁ and 50 ₂ illustratively control driveline torque ratherthan the position or pitch of the rollers 48 ₁ and 48 ₂. The rollers 48₁ and 48 ₂ are free-castoring, and are responsive to the actuators 50 ₁and 50 ₂ to seek a position that provides the correct ratio match ofengine and drive train speeds based on input energy equaling outputenergy.

In one illustrative implementation, the variator 40 includes two sets orpairs of disks 42 and 44, with the pairs of the disks 42 rigidly coupledto each other and with the pairs of the disks 44 also rigidly coupled toeach other, such that the embodiment illustrated in FIGS. 2A and 2Brepresents one-half of such an implementation. In this illustrativeimplementation, three rollers are positioned between each opposing setof disks 42, 44 for a total of six rollers 48 ₁-48 ₆ and sixcorresponding hydraulically controlled actuators 50 ₁-50 ₆. It will beunderstood, however, that this particular implementation of the variator40 is shown and described only by way of example, and that otherembodiments of the variator 40 that include more or fewer pairs of disks42, 44, that include more or fewer rollers 48 and hydraulicallycontrolled actuators 50, and/or that are configured to be only partiallytoroidal in shape, may alternatively be used. It will further beunderstood that while the operation of the variator 40 illustrated anddescribed herein as being generally hydraulically controlled, thisdisclosure contemplates embodiments in which operation of the variator40 is controlled via purely electronic or electro-mechanical structures.

Referring again to FIG. 1, the gear set within the combination variatorand gear set 20 illustratively includes one or more conventionalplanetary gear set(s) and/or other gear set(s) that define(s) at leasttwo automatically selectable gear ratios and that is coupled to, orintegrated with, the variator, e.g., the variator 40 illustrated anddescribed with respect to FIG. 2. The combination variator and gear set20 further illustratively includes a number of conventional frictiondevices, e.g., clutches, which may be selectively controlled to therebycontrol shifting of the transmission 14 between the two or more gearratios. In alternate embodiments, the gear set may include more than oneplanetary gear set, one or more planetary gear sets in combination withone or more other conventional gear sets, or exclusively one or morenon-planetary gear sets.

In the example embodiment illustrated in FIG. 1, the transmission 14includes three friction devices, e.g., in the form of three conventionalclutches C1, C2 and C3. In this embodiment, each clutch C1, C2 and C3 isoperated in a conventional manner, e.g., via fluid pressure, under thecontrol of the electro-hydraulic control system 24. In this regard, afluid path 25 ₁ is fluidly coupled between the electro-hydraulic controlsystem 24 and the clutch C1, a fluid path 25 ₂ is fluidly coupledbetween the electro-hydraulic control system 24 and the clutch C2, and afluid path 25 ₃ is fluidly coupled between the electro-hydraulic controlsystem 24 and the clutch C3. The gear set and the clutches C1, C2 and C3are illustratively arranged to provide four separate modes of operationof the transmission 14, and the various operating modes of thetransmission 14 are selectively controlled by the operation of theclutches C1, C2 and C3.

In a first operating mode, M1, for example, the clutch C1 is applied,e.g., engaged, while the clutches C2 and C3 are released, e.g.,disengaged, and in this mode forward or reverse launch can beaccomplished, and the vehicle carrying the transmission 14 can beoperated at vehicle speeds up to about 10 miles per hour. In a secondoperating mode, M2, as another example, the clutch C2 is engaged whilethe clutches C1 and C3 are disengaged, and in this mode the vehicle canbe operated at vehicle speeds in the range of about 10-30 miles perhour. In a third operating mode, M3, as yet another example, the clutchC3 is engaged while the clutches C1 and C2 are disengaged, and in thismode the vehicle can be operated at vehicle speeds greater than about 30miles per hour. In a fourth mode, M0, as a final example, the clutchesC1, C2 and C3 are all disengaged, and in this mode the transmission 14is in neutral. In one embodiment of the electro-hydraulic control system24 illustrated in FIG. 1, as will be described in greater detailhereinafter, two neutral conditions are possible; namely an in-rangeneutral and a so-called “true neutral.” In the transitional statesbetween the various operating modes M1, M2 and M3, the variator torqueis illustratively reversed to assist transitions from one operating modeto the next.

The system 10 further includes a transmission control circuit 30 thatcontrols and manages the overall operation of the transmission 14. Thetransmission control circuit 30 includes a number, M, of operatingparameter inputs, OP₁-OP_(M), that are electrically connected tocorresponding operating parameter sensors included within theelectro-hydraulic control system 24 via corresponding signal paths 26₁-26 _(M), wherein M may be any positive integer. The one or moreoperating parameter sensors included within the electro-hydrauliccontrol system 24, examples of which will be described hereinafter,produce corresponding operating parameter signals on the signal paths 26₁-26 _(M), which are received by the transmission control circuit 30.The transmission 14 further includes a number, N, of electricallycontrollable actuators included within the electro-hydraulic controlsystem 24 that are each electrically connected to different one of acorresponding number of actuator control outputs, AC₁-AC_(N) of thetransmission control circuit 30 via corresponding signal paths 28 ₁-28_(N), wherein N may be any positive integer. The one or moreelectrically controllable actuators included within theelectro-hydraulic control system 24, examples of which will be describedhereinafter, are responsive to actuator control signals produced by thetransmission control circuit 30 on the corresponding signal paths 28₁-28 _(N) to control various operational features of the transmission14.

Illustratively, the transmission control circuit 30 ismicroprocessor-based, and includes a memory unit 32 having instructionsstored therein that are executable by the control circuit 30 to controloperation of the transmission 14 generally, and more specifically tocontrol operation of the electro-hydraulic control system 24 as will bedescribed herein. It will be understood, however, that this disclosurecontemplates other embodiments in which the transmission control circuit30 is not microprocessor-based, but is configured to control operationof the transmission 14 generally and operation of the electro-hydraulicsystem 24 more specifically, based on one or more sets of hardwiredinstructions and/or software instructions stored in the memory unit 32.

Referring now to FIG. 3, a schematic diagram is shown of oneillustrative embodiment of the electro-hydraulic control system 24 ofFIG. 1. In the illustrated embodiment, the electro-hydraulic controlsystem 24 is roughly divided in two separate control sections; avariator control section 56 and a clutch control section 58. Aconventional fluid pump 60 is configured to supply transmission fluid,e.g., conventional transmission oil, to the variator control section 56from a source 64 of transmission fluid, e.g., a conventionaltransmission sump. In the illustrated embodiment, a fluid inlet of thefluid pump 60 fluidly coupled to the sump 64 via a fluid passageway 62.A fluid outlet of the pump 60 is fluidly coupled to an inlet of avariator main regulation block 66 via a variator main fluid passageway68 (VAM), and one of the output signal paths 28 ₁₀ of the controlcircuit 30 is electrically connected to the variator main regulationblock 66. The variator main regulation block 66 includes conventionalcomponents, e.g., one or more valves, responsive to a control signalproduced on the signal path 28 ₁₀ by the transmission control circuit 30to supply pressure-regulated transmission fluid to the fluid passageway68 in a conventional manner.

The variator main fluid passageway 68 is fluidly coupled to fluid inletsof two variator trim valves 70 and 72, to one end of a variator faultvalve 76 and also to a clutch control valve 96 located in the clutchcontrol section 58 of the electro-hydraulic control system 24. Thevariator trim valves 70 and 72 each include an actuator 78 and 84respectively that is electrically connected to the transmission controlcircuit 30 via a signal path 28 ₁ and 28 ₂ respectively. Another fluidinlet of each variator trim valve 70 and 72 is fluidly coupled toexhaust. A fluid outlet of the variator trim valve 70 is fluidly coupledto a variator control valve 82 via a fluid passageway 80, and a fluidoutlet of the variator trim valve 72 is fluidly coupled to anothervariator control valve 88 via a fluid passageway 86. In the illustratedembodiment, the actuators 78 and 84 are illustratively conventionalelectronically actuated solenoids, and the trim valves 70 and 72 areillustratively variable-bleed valves that supply variable-pressuretransmission fluid to the fluid passageways 80 and 86 respectively basedon control signals produced by the transmission control circuit 30 onthe signal paths 28 ₁ and 28 ₂ respectively.

The variator control section 56 of the electro-hydraulic control system24 further includes another variator trim valve 74 including an actuator90 that is electrically connected to the transmission control circuit 30via a signal path 28 ₃. One fluid inlet of the trim valve 74 is fluidlycoupled to the clutch control valve 96 via a fluid path 94, and anotherfluid inlet of the variator trim valve 74 is fluidly coupled to exhaust.A fluid outlet of the variator trim valve 74 is fluidly coupled to thevariator control valves 82 and 88 via a fluid passageway 92. Theactuator 90 illustratively a conventional electronically actuatedsolenoid, and the trim valve 74 is illustratively a variable-bleed valvethat supplies variable-pressure transmission fluid to the fluidpassageway 92 based on control signals produced by the transmissioncontrol circuit 30 on the signal path 28 ₃.

Another conventional fluid pump 98 is configured to supply transmissionfluid from the sump 64 to the clutch control section 58 of theelectro-hydraulic control system 24. In the illustrated embodiment, afluid inlet of the fluid pump 98 is fluidly coupled to the sump 64 viathe fluid passageway 62, and fluid outlet of the pump 98 is fluidlycoupled to a fluid inlet of a clutch and control main regulation, coolerand lube block 102 via a fluid passageway 100. Another one of the outputsignal paths 28 ₁₁ of the control circuit 30 is electrically connectedto the clutch and control main regulation, cooler and lube block 102.The clutch and control main regulation, cooler and lube block 102illustratively includes conventional components, e.g., one or morevalves, responsive to a control signal produced on the signal path 28 ₁₁by the transmission control circuit 30 to supply pressure-regulatedtransmission fluid to the clutch main, CLM, fluid passageway 100 and toa control main, COM, fluid passageway 104 in a conventional manner. Thecontrol main, COM, fluid passageway 104 is further fluidly coupled tothe variator control valves 82 and 88.

An exhaust backfill valve 106 establishes an exhaust backfill pressure,EB, in an exhaust backfill fluid passageway 108 that is also fluidlycoupled to the clutch and control main regulation, cooler and lube block102 and also to the variator fault valve 76. The clutch and control mainregulation, cooler and lube block 102 further includes conventionalcomponents for cooling and filtering the transmission fluid and forproviding lubrication paths to the variator and to the various gears ofthe gear set of the transmission 14.

The variator control valves 82 and 88 each include an actuator 85 and 95respectively that is electrically connected to the transmission controlcircuit 30 via a signal path 28 ₄ and 28 ₅ respectively. In theillustrated embodiment, the actuators 85 and 95 are illustrativelyconventional electronically actuated solenoids. The variator controlvalve 82 further includes a spool 110, and the actuator 85 is responsiveto control signals produced by the transmission control circuit 30 onthe signal path 28 ₄ to selectively control the position of the spool110 to thereby selectively control fluid pressure in a fluid passageway112. The variator control valve 88 likewise includes a spool 114, andthe actuator 95 is responsive to control signals produced by thetransmission control circuit 30 on the signal path 28 ₅ to selectivelycontrol the position of the spool 114 to thereby selectively controlfluid pressure in a fluid passageway 116. For purposes of this document,the fluid paths 112 and 116 may be referred to herein as S1 and S2respectively.

The S1 fluid path (112) is fluidly coupled to one end of a conventionaldamper 118, an opposite end of which is fluidly coupled to a variatorhigh-side fluid passageway 120. In the embodiment illustrated in FIG. 3,the variator includes six actuators, 50 ₁-50 ₆, e.g., conventionalpistons, and the variator high-side fluid passageway 120 is fluidlycoupled to one side, e.g., a high side, of each such actuator 50 ₁-50 ₆via a corresponding conventional damper 122 ₁-122 ₆. A conventionalcheck valve 124 is interposed between the variator high-side fluidpassageway 120 and the control main (COM) fluid path 104, and anotherconventional check valve 126 is interposed between the variatorhigh-side fluid passageway 120 and an endload fluid passageway 128.

The S2 fluid path (116) is similarly fluidly coupled to one end ofanother conventional damper 132, an opposite end of which is fluidlycoupled to a variator low-side fluid passageway 134. The variatorlow-side fluid passageway 134 is fluidly coupled to an opposite side,e.g., a low side, of each actuator 50 ₁-50 ₆ of the variator via acorresponding conventional damper 136 ₁-136 ₆. A conventional checkvalve 138 is interposed between the variator low-side fluid passageway134 and the control main (COM) fluid path 104, and another conventionalcheck valve 140 is interposed between the variator low-side fluidpassageway 134 and the endload fluid passageway 128. The endload fluidpassageway 128 is fluidly coupled to an endload relief valve 130, whichis further fluidly coupled between the high side and the low side of theactuator 50 ₆. Further details relating to one illustrative structureand method of operating the endload relief valve 130 are provided inco-pending U.S. Patent Application Ser. No. 61/287,020, the disclosureof which is incorporated herein by reference in its entirety.

The endload fluid passageway 128 is further fluidly coupled to anopposite end of the variator fault valve 76. The variator fault valve 76illustratively includes a spool 142, and is fluidly coupled to thevariator control valves 82 and 88 via a fluid passageway 144. The spool142 of the variator fault valve 76 is responsive to a difference inpressure between the variator main fluid passageway 68 at one end andthe endload fluid passageway 128 at its opposite end to supply aselectable fluid pressure to the fluid passageway 144. In the embodimentillustrated in FIG. 3, for example, if the fluid pressure in thevariator main fluid passageway 68 is sufficiently greater than that inthe endload fluid passageway 128, the spool 144 is forced upwardly andthereby fluidly couples the exhaust backfill fluid passageway (EB) 108to the fluid passageway 144. This is the position of the spool 142illustrated in FIG. 3. If instead the fluid pressure in the endloadfluid passageway 128 is sufficiently greater than that in the variatormail fluid passageway 68, the spool 144 is forces downwardly and therebyfluidly couples the control main (COM) fluid passageway 104 to the fluidpassageway 144. Illustratively, the variator fault valve 76 is designedto have a specified amount of hysteresis between the two extremepositions of the spool 142, and in one embodiment the hysteresis isapproximately 15-20% such that the differential pressure between VAM 68and the endload fluid passageway 128 must be greater than about 15-20%before the spool 142 changes position. Those skilled in the art willappreciate that this hysteresis value is provided only by way of exampleand that other hysteresis values, or no hysteresis value, mayalternatively be used.

In the illustrated embodiment, sensors are operatively positionedrelative to each of the variator control valves 82 and 88 to enablemonitoring of the operating states of each of these valves. In oneillustrative embodiment, the sensors are provided in the form ofconventional pressure switches, although it will be understood that aconventional pressure sensor may be substituted for any one or more ofthe pressure switches. In any case, each of the pressure switches iselectrically connected to the transmission control circuit 30 to allowmonitoring by the transmission control circuit 30 of the states of thepressure switches and thus the operating states of the valves 82 and 88.In the embodiment illustrated in FIG. 3, for example, a pressure switch146 is fluidly coupled to the variator control valve 82, and iselectrically connected to the transmission control circuit 30 via one ofthe signal paths 26 ₁. Another pressure switch 148 is fluidly coupled tothe variator control valve 88, and is electrically connected to thetransmission control circuit 30 via another one of the signal paths 26₂. The transmission control circuit 30 is operable to process thesignals produced by the pressure switch 146 and 148 in a known manner todetermine corresponding operating states, i.e., whether activated ordeactivated, of the valves 82 and 88. Further details relating to thestructure and operation of the variator control section 56 generally,and to the operation of and fault conditions associated with the valves70, 72, 74, 82 and 88 in particular, are provided in co-pending U.S.Patent Application Ser. No. 61/286,974, in co-pending U.S. PatentApplication Ser. No. 61/286,984, and in co-pending U.S. PatentApplication Ser. No. 61/287,003, the disclosures of which are allincorporated herein by reference in their entirety.

In the embodiment illustrated in FIG. 3, the clutch main pressure (CLM)is illustratively supplied via the fluid passageway 100 to the clutchcontrol section 58 of the electro-hydraulic control system 24. Inparticular, the clutch main fluid pressure, CLM, is fluidly applied viathe clutch main fluid passageway 100 is fluidly to each of a pair ofclutch trim valves 150 and 152. Together the clutch trim valves 150 and152 may be referred to herein as a trim system. The clutch trim valves150 and 152 each illustratively include an actuator 154 and 158respectively that is electrically connected to the transmission controlcircuit 30 via a signal path 28 ₆ and 28 ₇ respectively. One controlfluid inlet of each of the clutch trim valves 150 and 152 is fluidlycoupled to the control main fluid passageway 104, and another controlfluid inlet of each clutch trim valve 150 and 152 is fluidly coupled toexhaust. Each trim valve 150 and 152 further includes a movable spool156 and 160 respectively that is movable between two spool positionsbased on fluid pressure applied to control ends 156A and 160Arespectively thereof. In the illustrated embodiment, the actuators 154and 158 are illustratively conventional electronically actuatedsolenoids. The trim valves 150 and 152 are each configured toselectively supply control main (COM) pressure or exhaust to the controlends 156A and 160A of the spools 156 and 160 respectively based oncontrol signals produced by the transmission control circuit 30 on thesignal paths 28 ₆ and 28 ₇ respectively to thereby move the spools 156and 160 respectively between their two spool positions. The clutch trimvalves 150 and 152 are further fluidly coupled to each other via anumber of fluid passageways, and the exhaust backfill, EB, fluidpassageway 108 is fluidly coupled directly to the trim valve 150 andindirectly to the trim valve 152 via the trim valve 150.

Fluid outlets of each of the clutch trim valves 150 and 152 are fluidlycoupled to fluid inlets of each of a pair of clutch control valves 162and 96 via fluid passageways 172 and 174 respectively. The clutch trimvalves 150 and 152 are each configured to selectively, i.e., under thecontrol of the transmission control circuit 30 via signals produced bythe transmission control circuit 30 on the signal paths 28 ₆ and 28 ₇respectively, supply a clutch engagement pressure, e.g., the clutch mainpressure, CLM, and a clutch disengagement pressure, e.g., exhaustbackfill, EB, independently to the fluid passageways 172 and 174.

The clutch control valves 162 and 96 each illustratively include anelectronic actuator, e.g., an electrically controlled solenoid, 164 and168 respectively that is electrically connected to the transmissioncontrol circuit 30 via a signal path 28 ₈ and 28 ₉ respectively. Onecontrol fluid inlet of each clutch control valve 162 and 96 is fluidlycoupled to the control main, COM, fluid passageway 104, and anothercontrol fluid inlet is fluidly coupled to exhaust. Each valve 162 and 96is responsive to a control signal produced by the transmission controlcircuit 30 on the signal path 28 ₈ and 28 ₉ respectively to selectivelyapply the control main pressure, COM, or exhaust to a control end 166Aand 170A respectively of a spool 166 and 170 respectively carried byeach valve 162 and 96 to thereby move the spools 166 and 170 between twospool positions. The clutch control valves 162 and 96 are furtherfluidly coupled to each other via fluid passageways 176, 178, 180 and182. The control main pressure, COM, fluid passageway 104 is alsofluidly coupled directly to the other portions of each clutch controlvalve 162 and 96, and the exhaust backfill, EB, fluid passageway 108 isfluidly coupled directly to each of the clutch control valves 162 and96.

The clutch control valve 96 is further fluidly coupled directly to theC2 clutch fluid path 25 ₂, and clutch main fluid, CLM, or exhaustbackfill, EB, is selectively applied to the C2 clutch via the fluid path25 ₂ via various combinations of states of the actuators 154, 158, 164and 168. The clutch control valve 162 is further fluidly coupleddirectly to each of the C1 and C3 clutch fluid paths 25 ₁ and 25 ₃, andclutch main fluid, CLM, or exhaust backfill, EB, is selectively routedthrough the clutch control valve 162 to the C1 clutch via the fluidpassageway 25 ₁ or to the C3 clutch via the fluid passageway 25 ₃ viavarious combinations of states of the actuators 154, 158, 164 and 168.The clutches C1-C3 are thus selectively activated, i.e., engaged, anddeactivated, i.e., disengaged, based on the operating states of theactuators 154, 158, 164 and 168 of the clutch trim valves 150 and 152and the clutch control valves 162 and 96 respectively, by selectivelyrouting the CLM and EB pressures through the control valves 162 and 96to the various clutches C1-C3. The clutch control valve 96 is directlyfluidly coupled to the clutch C2 via the fluid passageway 25 ₂, andcontrol, i.e., engagement and disengagement, of the C2 clutch musttherefore include appropriate control of the clutch control valve 96 toselectively route the CLM and EB pressures to the clutch C2. The clutchcontrol valve 162, on the other hand, is directly fluidly coupled to theclutches C1 and C3 via the fluid passageways 25 ₁ and 25 ₃ respectively,and control, i.e., engagement and disengagement, of the clutches C1 andC3 must therefore include appropriate control of the clutch controlvalve 162 to selectively route the CLM and EB pressures to the clutchesC1 and C3. Because the clutches C1 and C3 are never, during normaloperation of the transmission 14, engaged simultaneously, control of theclutches C1 and C3 can therefore be multiplexed via the clutch controlvalve 162.

In the illustrated embodiment, sensors are operatively positionedrelative to the clutch trim valves 150 and 152 and each of the clutchcontrol valves 162 and 96 to enable monitoring of the operating statesof each of the valves 150, 152, 162 and 96 and to further monitorcertain transmission operating state faults. In one illustrativeembodiment, such sensors are provided in the form of conventionalpressure switches, although it will be understood that a conventionalpressure sensor may be substituted for any one or more of the pressureswitches. In any case, each of the pressure switches is electricallyconnected to the transmission control circuit 30 to allow monitoring bythe transmission control circuit 30 of the states of the pressureswitches and thus the operating states of the each of the valves 150,152, 162 and 96 and of certain transmission operating state faults. Inthe embodiment illustrated in FIG. 3, for example, a pressure switch 184is fluidly coupled to the clutch control valve 162, and is electricallyconnected to the transmission control circuit 30 via one of the signalpaths 26 ₃. Another pressure switch 186 is fluidly coupled to the clutchtrim valves 150 and 152, and is electrically connected to thetransmission control circuit 30 via another one of the signal paths 26₄. Still another pressure switch 188 is fluidly coupled to the clutchcontrol valve 96, and is electrically connected to the transmissioncontrol circuit 30 via another one of the signal paths 26 ₅. Thetransmission control circuit 30 is operable to process the signalsproduced by the pressure switches 184, 186 and 188 to determinecorresponding operating states, i.e., whether activated or deactivated,of the various valves 150, 152, 162 and 96 and of certain transmissionoperating state faults. Further details relating to methods forprocessing the signals produced by the pressure switches 184, 186 and188 to monitor fault states associated with the valves 152, 162 and 96and to monitor certain transmission operating state faults are providedin co-pending U.S. Patent Application Ser. No. 61/287,031, thedisclosure of which is incorporated herein by reference in its entirety.

Referring now to FIGS. 4-7, magnified views of the four possibleoperating states of the clutch control valves 162 and 96 are shown.During normal operation of the transmission 14 in any of the operatingmodes M1-M3 described above, except during neutral, one of the clutchesC1-C3 is engaged. However, during mode transitions, e.g., 1-2 or 2-3,two clutches are simultaneously engaged for at least a short time periodwhile the oncoming clutch engages and the off-going clutch disengages.During such mode transitions, both of the clutch trim valves 150 and 152are activated, i.e., stroked, such that the clutch trim valve 150supplies the clutch main pressure, CLM, to the fluid passage 172 and theclutch trim valve 152 likewise supplies the clutch main pressure, CLM,to the fluid passage 174.

Under such conditions, i.e., when the fluid passages 172 and 174 bothcarry the clutch main pressure, CLM, FIG. 4 illustrates the case for anormal Mode 1-to-Mode 2 transition in which both of the clutch controlvalves 162 and 96 are activated, i.e., stroked, such that the controlmain pressure, COM, is applied by the actuator 164 to the control end166A of the valve spool 166 and also by the actuator 168 to the controlend 170A of the valve spool 170. In the resulting stroked position ofthe spool 166, the clutch control valve 162 fluidly couples the fluidpassageway 172 to the C1 clutch fluid passageway 25 ₁, thereby engagingthe C1 clutch. Likewise, in the resulting stroked position of the spool170, the clutch control valve 96 fluidly couples the fluid passageway174 to the C2 clutch fluid passageway 25 ₂, thereby engaging the C2clutch. The stroked positions of the spools 166 and 170 further fluidlycouple the exhaust backfill passageway 108, via the fluid passageway180, to the C3 clutch fluid passageway 25 ₃, thereby exhausting anddisengaging the C3 clutch. The control main pressure, COM, is routed bythe stroked positions of the spools 166 and 170 to both of the pressureswitches 184 and 188. The transmission control circuit 30 processes thesignals produced by the pressure switches 184 and 188 on the signalpaths 26 ₃ and 26 ₅ respectively, and accordingly interprets thepressure switch states as “1 1” thereby identifying both of the clutchcontrol valves 162 and 96 respectively as activated or stroked. Thisclutch valve position is typically used for launch, reverse and lowerforward speeds. The control main pressure, COM, is also routed by thestroked position of the spool 170 to the variator trim valve 90 via thefluid passageways 176 and 94.

FIG. 5 illustrates the case for a normal Mode 2-to-Mode 3 transition inwhich the clutch control valve 162 is deactivated, i.e., de-stroked, andthe clutch control valve 96 is activated, i.e., stroked, such that theactuator 164 exhausts the control end 166A of the valve spool 166, andcontrol main pressure, COM, is applied by the actuator 168 to thecontrol end 170A of the valve spool 170. In the resulting deactivated orde-stroked position of the spool 166, the clutch control valve 162fluidly couples the fluid passageway 172, via the fluid passageway 182,to the C3 clutch fluid passageway 25 ₃, thereby engaging the C3 clutch.In the resulting stroked position of the spool 170, the clutch controlvalve 96 fluidly couples the fluid passageway 174 to the C2 clutch fluidpassageway 25 ₂, thereby engaging the C2 clutch. The de-stroked positionof the spool 166 and the stroked position of the spool 170 furtherfluidly couple the exhaust backfill passageway 108, via the fluidpassageway 180, to the C1 clutch fluid passageway 25 ₁, therebyexhausting and disengaging the C1 clutch. The exhaust backfill pressure,EB, is routed by the de-stroked position of the spool 166 to thepressure switch 184, and the control main pressure, COM, is routed bythe stroked positions of the spool 170 to the pressure switch 188. Thetransmission control circuit 30 processes the signals produced by thepressure switches 184 and 188 on the signal paths 26 ₃ and 26 ₅respectively, and accordingly interprets the pressure switch states as“0 1” thereby identifying the clutch control valve 162 as deactivated orde-stroked and the clutch control valve 96 as activated or stroked. Thisclutch valve position is typically used for higher forward speeds. Thecontrol main pressure, COM, is again routed by the stroked position ofthe spool 170 to the variator trim valve 90 via the fluid passageways176 and 94.

FIG. 6 illustrates one of two clutch control valve states that generallywould not occur during normal Mode-to-Mode transitions, and thereforerepresents one fault or failure state of the clutch control valves. Morespecifically, the case illustrated in FIG. 6 has the clutch controlvalve 162 activated, i.e., stroked, and the clutch control valve 96deactivated, i.e., de-stroked, such that control main pressure, COM, isapplied by the actuator 164 to the control end 166A of the valve spool166, and the actuator 168 exhausts the control end 170A of the valvespool 170. In the resulting activated or stroked position of the spool166, the clutch control valve 162 fluidly couples the fluid passageway172 to the C1 clutch fluid passageway 25 ₁, thereby engaging the C1clutch as was illustrated in FIG. 4. In the resulting de-strokedposition of the spool 170, the clutch control valve 96 fluidly couplesthe fluid passageway 174, via the fluid passageway 180, to the C3 clutchfluid passageway 25 ₃, thereby also engaging the C3 clutch. The strokedposition of the spool 166 and the de-stroked position of the spool 170further fluidly couple the exhaust backfill passageway 108, via thefluid passageway 178, to the C2 clutch fluid passageway 25 ₂, therebyexhausting and disengaging the C2 clutch. The control main pressure,COM, is routed by the stroked position of the spool 166 to the pressureswitch 184, and the control main pressure, COM, routed to the pressureswitch 188 is exhausted at the opposite end of the spool 170 as a resultof the de-stroked position of the spool 170. The transmission controlcircuit 30 processes the signals produced by the pressure switches 184and 188 on the signal paths 26 ₃ and 26 ₅ respectively, and accordinglyinterprets the pressure switch states as “1 0” thereby identifying theclutch control valve 162 as activated or stroked and the clutch controlvalve 96 as deactivated or de-stroked. This clutch valve position wouldgenerally not occur during normal operation of the transmission 14 as itsimultaneously engages the clutches C1 and C3, and thus corresponds to afault or failure condition. However, it should be noted that althoughthe clutch valve position illustrated in FIG. 6 corresponds to a faultor failure condition, the clutch control valves 162 and 96 have beenconfigured to provide limp-home capability by ensuring engagement of atleast one of the clutches C1-C3. The variator main pressure, VAM, isrouted by the de-stroked position of the spool 170 to the variator trimvalve 90 via the fluid passageway 94.

FIG. 7 illustrates the other of two clutch control valve states thatgenerally would not occur during normal Mode-to-Mode transitions, andtherefore represents another fault or failure state of the clutchcontrol valves. More specifically, the case illustrated in FIG. 7 hasboth the clutch control valve 162 and the clutch control valve 96deactivated, i.e., de-stroked, such that the actuator 164 exhausts thecontrol end 166A of the valve spool 166 and the actuator 168 exhauststhe control end 170A of the valve spool 170. In the resultingdeactivated or de-stroked positions of the spools 166 and 96, the clutchcontrol valve 162 fluidly couples the fluid passageway 172 to fluidpassageway 178, and the clutch control valve 96 fluidly couples thefluid passageway 178 to the C2 clutch fluid passageway 25 ₂, therebyengaging the C2. The de-stroked position of the spool 170 of the clutchcontrol valve 96 further fluidly couples the fluid passageway 174, viathe fluid passageway 180, to the C1 clutch fluid passageway 25 ₁,thereby also engaging the C1 clutch. The de-stroked positions of thespool 166 and 170 further fluidly couple the exhaust backfill passageway108 to the C3 clutch fluid passageway 25 ₃, thereby exhausting anddisengaging the C3 clutch. The exhaust backfill, EB, is further routedby the de-stroked positions of the spools 166 and 170 to both of thepressure switches 184 and 188. The transmission control circuit 30processes the signals produced by the pressure switches 184 and 188 onthe signal paths 26 ₃ and 26 ₅ respectively, and accordingly interpretsthe pressure switch states as “0 0” thereby identifying the clutchcontrol valve 162 as deactivated or de-stroked and the clutch controlvalve 96 as deactivated or de-stroked. This clutch valve position wouldgenerally not occur during normal operation of the transmission 14, andthus corresponds to a fault or failure condition. However, it should benoted that although the clutch valve position illustrated in FIG. 7corresponds to a fault or failure condition, the clutch control valves162 and 96 have been configured to provide limp-home capability byensuring engagement of at least one of the clutches C1-C3. As was thecase in FIG. 6, the variator main pressure, VAM, is routed in FIG. 7 bythe de-stroked position of the spool 170 to the variator trim valve 90via the fluid passageway 94.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An apparatus for multiplexing gear engagement control in an automatic transmission operable in a plurality of operating modes, the apparatus comprising: a first and a second friction engagement device, each of the first and second friction engagement devices selectively engageable and disengageable to select a different gear ratio of the transmission, a trim system configured to selectively supply fluid at one of (i) an engagement pressure and (ii) a disengagement pressure to a first fluid passageway and a second fluid passageway in one of the plurality of operating modes, and a first control valve fluidly coupled directly to the first fluid passageway and the second fluid passageway and directly to the first friction engagement device and the second friction engagement device, wherein the first control valve is configured to contemporaneously route fluid at the engagement pressure to the first friction engagement device to engage the first friction engagement device and to the second friction engagement device to engage the second friction engagement device in a fault condition occurring during a transition between the plurality of operating modes of the transmission.
 2. The apparatus of claim 1, wherein the first control valve is configured to (i) selectively route fluid at the engagement pressure to the first friction engagement device to the engage the first friction engagement device in one of the plurality of operating modes, and (ii) contemporaneously route fluid at the disengagement pressure to the second friction engagement device to disengage the second friction engagement device in the one of the plurality of operating modes.
 3. The apparatus of claim 2, further comprising a second control valve fluidly coupled directly to each of the first and second fluid passageways, the second control valve configured to selectively route the engagement pressure in one of the first and second fluid passageways and the disengagement pressure in one of the first and second fluid passageways through the second control valve to the first control valve for further selective routing by the first control valve to each of the first and second friction engagement devices.
 4. The apparatus of claim 3, further comprising a third friction engagement device selectively engageable and disengageable to select another different gear ratio of the transmission, wherein the second control valve is fluidly coupled directly to the third friction engagement device, and wherein the second control valve is configured to selectively route one of the engagement and disengagement pressures in the second fluid passageway through the second control valve directly to the third friction engagement device.
 5. The apparatus of claim 4, wherein the first control valve comprises a first spool having a stroked position and a de-stroked position and the second control valve comprises a second spool having a stroked position and a de-stroked position, and wherein the first and second control valves are together configured to supply the engagement pressure in at least one of the first and second fluid passageways to at least one of the first, second, and third friction engagement devices to thereby engage the at least one of the first, second, and third friction engagement devices in all possible position combinations of the first and second spools.
 6. The apparatus of claim 5, wherein the first spool of the first control valve has a stroked position in the fault condition, and wherein the second spool of the second control valve has a de-stroked position in the fault condition.
 7. The apparatus of claim 5, wherein the second control valve is fluidly coupled directly to a first main pressure fluid passageway and also directly to a third fluid passageway, the second control valve configured to selectively route pressure in the first main pressure fluid passageway to the third fluid passageway.
 8. The apparatus of claim 6, wherein the second control valve routes fluid at the disengagement pressure to the third friction engagement device to disengage the third friction engagement device in the fault condition.
 9. The apparatus of claim 7, wherein the second control valve is fluidly coupled directly to a second main pressure fluid passageway, the second control valve configured to selectively route pressure in the second main pressure fluid passageway to the third fluid passageway.
 10. The apparatus of claim 9, wherein the automatic transmission is a toroidal traction drive transmission further comprising a variator and a variator control system for controlling operation of the variator, and wherein the second control valve is configured to selectively route pressure in the first and second main pressure fluid passageways to a component of the variator control system via the third fluid passageway.
 11. An apparatus for multiplexing gear engagement control in an automatic transmission operable in a plurality of operating modes, the apparatus comprising: a first control valve fluidly coupled to a first fluid passageway, a second fluid passageway, a first friction engagement device, and a second friction engagement device, wherein the first control valve is configured to contemporaneously route fluid at (i) an engagement pressure to the first friction engagement device to engage the first friction engagement device and (ii) a disengagement pressure to the second friction engagement device to disengage the second friction engagement device in a fault condition occurring during a transition between the plurality of operating modes of the transmission.
 12. The apparatus of claim 11, further comprising a second control valve fluidly coupled directly to each of the first and second fluid passageways, the second control valve configured to selectively route the engagement pressure in one of the first and second fluid passageways and the disengagement pressure in one of the first and second fluid passageways through the second control valve to the first control valve for further selective routing by the first control valve to each of the first and second friction engagement devices.
 13. The apparatus of claim 12, wherein the second control valve is fluidly coupled directly to a third friction engagement device, and wherein the second control valve is configured to selectively route one of the engagement and disengagement pressures in the second fluid passageway through the second control valve directly to the third friction engagement device.
 14. The apparatus of claim 13, wherein the first control valve comprises a first spool having a stroked position and a de-stroked position and the second control valve comprises a second spool having a stroked position and a de-stroked position, and wherein the first and second valves are together configured to supply the engagement pressure in at least one of the first and second fluid passageways to at least one of the first, second, and third friction engagement devices to thereby engage the at least one of the first, second, and third friction engagement devices in all possible position combinations of the first and second spools.
 15. The apparatus of claim 14, wherein the first spool of the first control valve has a de-stroked position in the fault condition, and wherein the second spool of the second control valve has a de-stroked position in the fault condition.
 16. The apparatus of claim 15, wherein the second control valve is configured to route fluid at the engagement pressure to the third friction engagement device to engage the third friction engagement device in the fault condition.
 17. The apparatus of claim 16, wherein a trim system is configured to supply the engagement pressures to each of the first and second fluid passageways during the transition between the plurality of operating modes of the transmission to thereby engage two of the first, second, and third friction engagement devices during the transition.
 18. The apparatus of claim 17, wherein the de-stroked and stroked positions of the first spool of the first control valve and the de-stroked and stroked positions of the second spool of the second control valve define four separate position combinations of the first and second control valves, and wherein two of the four separate position combinations of the first and second control valves are possible during transitions between the plurality of operating modes of the transmission.
 19. The apparatus of claim 18, wherein at least one of the remaining two of the four separate position combinations of the first and second control valves represent the fault condition. 